Detection beyond the standard radiation noise limit using reduced emissivity and optical cavity coupling

ABSTRACT

The present invention provides thermal detectors having an optical cavity that is optimized to couple light into a sensor. Light that is on resonance is coupled with the sensor with as high as 100% efficiency, while light off resonance is substantially reflected away. Light that strikes the sensor from the sides (i.e. not on the optical cavity axis) only interacts minimally with sensor because of the reduced absorption characteristics of the sensor. Narrowband sensors in accordance with the present invention can gain as much as 100% of the signal from one direction and spectral band, while receiving only a fraction of the normal radiation noise, which originates from all spectral bands and directions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional ApplicationNo. 60/873,650, filed Dec. 8, 2006, the entire contents of which areincorporated herein by reference for all purposes.

GOVERNMENT RIGHTS

This invention was made with Government support under Grant No.DAAD19-03-1-3043, awarded by the Army Research Office.

TECHNICAL FIELD

The present invention is directed to infrared detectors and relatedmethods. In particular, the present invention is directed to highsensitivity uncooled thermal detectors that function with noise floorsbelow the standard blackbody radiation limit. Thermal detectors inaccordance with the present invention include microbolometers baseddetectors as well as ferroelectric, pyroelectric, and thermoelectricdetectors. The techniques described herein can also benefit photon(e.g., p-n junctions and photoconductors) based detectors under certainconditions.

BACKGROUND

Thermal detectors include a sensor that absorbs light energy and thentransduces the resulting heat into a useful electrical signal related tothe amount or type of light absorbed. Perhaps the most prominent currentthermal detectors include microbolometers, which absorb light across abroad band of the infrared, usually the mid-wave infrared (MWIR,corresponding to wavelengths of roughly 3 microns to 5 microns) or longwave infrared (LWIR, roughly 8 microns to 14 microns) and then convertheat into a change in resistance. These devices are very popular incommercial uncooled imaging cameras. Their basic structure includes asmall micromachined sensor plate connected to an underlying substrate bythin support beams. The support beams have a low thermal conductance sothat large increases in the temperature of the sensor plate occur withsmall amounts of absorbed light. The sensor plate includes a resistormade of a material with a high magnitude temperature coefficient ofresistance (TCR). One common TCR material in use is vanadium oxide,originally developed for microbolometers in the 1980's. A pulsed orcontinuous bias current is applied to the resistor and the absorbedlight energy can be measured through the voltage response. Some othercommon thermal detector technologies include thermoelectric detectorswhere the heat from light is converted into a voltage using the Seebeckeffect, and pyroelectric detectors where heat from absorbed lightinduces a voltage signal via a change in the internal polarization of aferroelectric material.

There are a variety of noise sources that can limit the performance of athermal detector. For a biased single-pixel detector, the most importantof these include Johnson noise, 1/f noise, and thermal noise. Thermalnoise originates from the fluctuations in the quanta of energytransferred to and from the detector. These quanta can take the form ofeither phonons if solid-state conduction dominates the heat transfer orphotons if radiation dominates. Traditionally, radiation heat transferhas been considered the fundamental noise limit because even if all ofthe other noise sources are reduced by technological innovations, thephoton fluctuations still remain due to Planck's Law.

Current broadband thermal detector devices are beginning to perform inregimes where radiation noise must be considered. An example of this isshown in U.S. Publication Nos. 2002/0139933 and 2001/0028035 whichdescribe a microbolometer with low thermal conductance supports. In oneembodiment, the authors propose to select a material for the backside oftheir detector which has a radiation emission lower than many othermaterials.

The radiation limit is more forgiving than has been previouslyconsidered. Because radiation heat transfer is directly proportional toemissivity and emissivity is identical to absorption through Kirchoff'sLaw, a low absorption structure will interact with the radiation of thebackground much less than a normal thermal detector, which is usuallyoptimized for high absorption. For a typical detector, this does nothingto improve performance because the received optical signal is reduced bythe same amount. If the signal light can be coupled into the sensor ofthe detector at near 100% efficiency while maintaining a low absorptionfor the rest of the background, then the traditional thermal radiationnoise could be reduced by multiple orders of magnitude.

Most thermal detectors operate with broad bands. Many current deviceshave a reflector beneath them to allow light transmitted through thedevice another chance to be absorbed. The gaps between the bolometer andsubstrate in these structures are roughly a quarter-wave length toenhance coupling across a wide band. In a sense, the entire bolometerplus substrate can be though of as a highly absorbing distributed“mirror.” A variety of narrowband detectors using integrated externaletalons have been proposed, perhaps the most advanced of which aredescribed by in U.S. Pat. Nos. 7,015,457 and 5,550,373 to Cole et al. Amultispectral bolometer concept that uses scattering rather thaninterference to distribute different wavelengths is described in U.S.Pat. No. 5,629,521 to Lee et al. Detectors with integrated filtering aredescribed in U.S. Pat. No. 5,589,689 to Koskinen, U.S. Publication No.2005/0017177 to Tai et al., and U.S. patent application Ser. No.11/805,240 to Talghader et al.

SUMMARY

The present invention provides high sensitivity thermal detectors thatperform far beyond the blackbody radiation noise limit. Thermaldetectors in accordance with the present invention comprise a lowemissivity (absorption) sensor such as a microbolometer plate, forexample, positioned within an optical cavity. The optical cavity is atleast partially defined by thin-film mirror structures. The absorptionof the sensor is preferably matched to the reflectivity of the cavitymirrors thereby optimizing coupling of radiation with the sensor.Curvature of the mirrors is also preferably optimized to minimize thearea of the sensor while maximizing the amount of radiation collected bythe sensor. Radiation that is on resonance is coupled with the sensor atas high as 100% efficiency, while radiation off resonance is reflectedaway. Advantageously, radiation that strikes the sensor from the sides(i.e. not on the optical cavity axis) only interacts minimally with thesensor because of the reduced absorption characteristics of the sensor.Narrowband thermal detectors in accordance with the present inventioncan gain as much as 100% of the signal from one direction and spectralband, while receiving only a fraction of the normal radiation noise,which originates from all spectral bands and directions.

Thermal detectors in accordance with the present invention have a noisefloor below the broadband thermal radiation limit. For current state ofthe art microbolometer based thermal detectors, typical noise powers forthermal radiation noise vary from picowatts to tens of picowattsdepending on the area of the detector. Area for area, microbolometerbased thermal detectors in accordance with the present invention achievenoise powers an order of magnitude better than the state of the artdetectors.

Thermal detectors in accordance with the present invention alsopreferably utilize ultra-low thermal conductance support structures andprovide sub-radiation limit sensitivity for large area devices or smallsingle pixels. In one embodiment supports comprising a dielectricstructural layer and conductor are preferably used. In the current stateof the art, these supports can have thermal conductances on the order of10⁻⁹ W/K with resistances on the order of 100 kΩ or less. In general,when using the same materials, a higher thermal conductance leads to alower electrical resistance. Some preferred support materials includelow thermal conductivity dielectrics such as silicon dioxide andmagnetic metal alloys such as nickel-iron (NiFe), for example. Fordetectors using continuous bias read-out and similar read-outs with lowJohnson and 1/f noise, low thermal conductivity support structures canbe used to achieve sensitivity beyond the standard blackbody radiationlimit. However, to obtain array compatible small pixels at maximumsensitivity using pulse bias read-out, more advanced support structuressuch those using thermal switching and/or interface contacts arepreferably used. Sensitive read-outs may use optical means, say bytracking the spectral position of the cavity optical resonance or thespatial position of the pattern diffracted, transmitted, refracted, orreflected from the sensor by a read-out optical probe. While opticaltechniques are more difficult with imaging arrays, such techniques havethe advantage of reducing or eliminating electrical read-out noise.

Sensors in accordance with the present invention preferably comprisethin membranes, not only for emissivity reasons, but also for optimizingtime constant. In order to operate in the thermal radiation limit thethermal conductance must be extremely low, which forces the timeconstant of the sensors to undesirably high values. To circumvent this,sensor membranes are preferably reduced in thickness to a few tens ofnanometers or less. In this way, typical frame rate detection can stillbe achieved. It should be noted that having long time constants in somepotential applications such as chemical sensing or astronomy is notnecessarily detrimental, so the exact thickness desired will be afunction of desired time constant, sensor area, and materials used.

Exemplary sensor materials comprise VO_(x) and SiO₂. VO_(x) is one ofthe industry standard TCR materials, having been used since at least themid-80's in microbolometers, and as a result has a well-characterizedhigh-TCR response. The SiO₂ is used as an etch stop and structuralmaterial. VO_(x) can be deposited to have minimal absorption in theLWIR, and SiO₂, with a strong absorption near 10 microns, can bedeposited to a thickness chosen to give a desired emissivity near thatwavelength. Other materials can be used for other wavelengths or forwavelength-tunable devices. For example, a thin metal can be used in anabsorber if a tunable device were desired, because metals commonly havea more uniform spectral absorption across the LWIR or MWIR as comparedto SiO₂ and therefore would couple properly to an entire range ofoptical cavity resonances. For the supports, materials having lowthermal conductivity and conductive layers with a high ratio ofelectrical to thermal conductivity are preferred. Exemplary materialsinclude silicon dioxide as the structural support material and sputteredNiFe as the conductor.

The present invention also provides pixel structures, for example, forimaging chemical lines at high sensitivity. Such pixel structures have anoise equivalent power (NEP) that is at least one order of magnitudelower than an equivalent state of the art microbolometer based detectordue to the reduced interaction with the radiation background provided bydetectors of the present invention.

Additionally, thermal detectors in accordance with the present inventioncan be made tunable by using an electrostatic actuator that controls therelative spacing of the mirrors and sensor.

In an aspect of the present invention, a thermal detector for sensinginfrared radiation is provided. The thermal detector comprises aresonant optical cavity at least partially defined by first and secondspaced apart layered thin-film mirror structures; and an infrared sensoroperatively positioned between the first and second thin-film mirrorstructures and suspended within the resonant optical cavity by one ormore electrically conductive support beams so radiation received by theresonant optical cavity impinges on at least a surface portion of theinfrared sensor. The optical cavity may include plural sensors.

In another aspect of the present invention, a thermal detector forsensing infrared radiation is provided. The thermal detector comprises aresonant optical cavity at least partially defined by first and secondspaced apart layered thin-film mirror structures; and an infrared sensoroperatively positioned between the first and second thin-film mirrorstructures and suspended within the resonant optical cavity by one ormore electrically conductive support beams so radiation received by theresonant optical cavity impinges on at least a portion of the infraredsensor and wherein said portion of the infrared sensor has an absorptionless than or equal to twenty-five percent. Preferably, said portion ofthe infrared sensor has an absorption less than or equal to twenty-fivepercent in the range of about 8 microns to about 14 microns at roomtemperature. This wavelength range will move shorter at higherbackground temperatures.

In another aspect of the present invention, a method of sensing infraredradiation is provided. The method comprises the steps of providing aresonant optical cavity at least partially defined by first and secondspaced apart mirror structures and an infrared sensor suspended withinthe resonant optical cavity; causing radiation to enter the resonantoptical cavity and couple with the infrared sensor; and measuring achange in the infrared sensor caused by the radiation coupled with theinfrared sensor.

In yet another aspect of the present invention, a method of making athermal detector for sensing infrared radiation is provided. The methodcomprises the steps of forming a resonant optical cavity at leastpartially defined by first and second spaced apart thin-film mirrorstructures; suspending an infrared sensor within the resonant opticalcavity; and positioning the infrared sensor within the optical cavity sothe absorption of the resonant wavelength of the optical cavity by thesensor is greater than twenty-five percent.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this application, illustrate several aspects of the inventionand together with description of the embodiments serve to explain theprinciples of the invention. A brief description of the drawings is asfollows:

FIG. 1 is a schematic view of an exemplary ultra-high sensitivitynarrowband thermal detector having a Gaussian optical cavity and acentral absorbing sensor in accordance with the present invention. Thesensor has low emissivity and therefore interacts strongest withradiation that is in the frequency band and direction of the opticalcavity.

FIG. 2 is a table showing an exemplary layer structure and thicknessesfor a cavity-enhanced absorption sensor in accordance with the presentinvention.

FIG. 3 is a graph showing absorbance versus wavelength for asub-radiation-limit detector resonant optical cavity in accordance withthe present invention having a layer structure similar to the layerstructure of the table of FIG. 2.

FIG. 4 is a table showing layer structure of a resonant optical cavityin accordance with the present invention having mirror structurescomprising Ge/SrF₂. In this optical cavity structure, the sensor iscentered within the optical cavity.

FIG. 5 is a graph of absorption versus wavelength for the resonantoptical cavity of the table of FIG. 4.

FIG. 6 is a conceptual diagram of the change in emissivity andabsorptivity that results from a mostly transparent object.

FIG. 7 is a graph of detectivity vs. wavelength for a thermal detectorin accordance with the present invention having a chromium absorberlayer inside an optical cavity optimized for ten microns.

FIG. 8 is a schematic view of an exemplary thermal detector inaccordance with the present invention.

FIG. 9 is a schematic view of a sensor structure of the thermal detectorof FIG. 8.

FIG. 10 is a top view of the thermal detector of FIG. 8.

FIG. 11 is a partial exploded view of the thermal detector of FIG. 8showing a mirror structure and sensor structure.

FIG. 12 is an exploded view of the thermal detector of FIG. 8 showingfirst and second mirror structures and a sensor structure.

FIG. 13 is a schematic view of another exemplary thermal detector inaccordance with the present invention.

FIG. 14 is a schematic view of another exemplary thermal detector inaccordance with the present invention.

FIG. 15 is a schematic view of another exemplary thermal detector inaccordance with the present invention.

FIG. 16 is a top view of the thermal detector of FIG. 15.

FIG. 17 is a top view of a sensor structure of the thermal detector ofFIG. 15.

FIG. 18 is a schematic view of an exemplary photon detector inaccordance with the present invention.

FIG. 19 is a table showing layer structure of a resonant optical cavityin accordance with the present invention having mirror structurescomprising Ge/SrF₂. In this optical cavity structure, the sensor is notcentered within the optical cavity.

FIG. 20 is a graph of absorption versus wavelength for the resonantoptical cavity of the table of FIG. 19.

DETAILED DESCRIPTION

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention.

The radiation limit is widely known to dictate the ultimate performanceof thermal detectors. Even if all other noise sources are eliminated orreduced to negligible levels, the transfer of photons between thedetector, its surroundings, and the background will cause a minimumfluctuation in detected power. The present invention thus providesdetectors having low enough emissivity (absorption) so that a sensorportion of the detector interacts only weakly with its surroundings.Normally, this is unacceptable because the sensor signal responsedecreases along with the noise. However, if the sensor is placed insidea properly matched optical cavity, then light in one direction and onefrequency band will couple at 100% or near 100% to the sensor. Otherwavelengths of radiation in the direction of the cavity axis will berejected by the cavity, and radiation of any wavelength from otherdirections will interact only weakly with the low-absorption sensorelement. With this type of device, the radiation noise limit can bedropped by two or more orders of magnitude, making narrowband uncooleddetectors perform almost as well as the best cooled narrowband IndiumAntimonide or Mercury Cadmium Telluride (MCT) photon detectors. Thetechnology applies to both fixed-wavelength and tunable detectors.Tunable detectors are described in U.S. patent application Ser. No.10/805,240 to Talghader et al., entitled “Tunable Finesse InfraredCavity Thermal Detectors,” and filed on 22 May 2007, the entiredisclosure of which is incorporated by reference herein for all itsentire disclosure and for all purposes.

Detection beyond the standard radiation noise limit can be extended towork at broader resonances to some degree; however, this process becomesless efficient the wider the band. To achieve broad response, themirrors are made less reflective and the sensor more absorbing. Thesechanges increase the interaction of the sensor with the background andreduce the ultimate performance of the device. In the trivial case, theperformance enhancement decreases toward zero as one tries to cover theentire MWIR or LWIR. One can remove one mirror entirely and make thesensor highly absorbing to mimic architectures common to currentmicrobolometers where a reflection from the substrate or other layer isused to couple extra radiation into the sensor (see U.S. Pat. Nos.5,021,663 and 5,286,976 and U.S. Publication No. 2002/0179837, forexample). Such devices have radiation noise near the traditionalblackbody limit.

It is well known that two parallel mirrors separated by a multiple of ahalf wavelength will produce a Fabry-Perot optical cavity that has atransmission peak at that wavelength. The same interference phenomenathat lead to high transmission can also be used to couple radiation intoan absorbing layer in the resonant cavity. If a weakly absorbing layeris placed at a maximum of the field intensity inside a high Finessecavity, then radiation on resonance will be strongly absorbed because itmakes many passes through the absorber material. Radiation off resonanceis rejected by the optical cavity itself and interacts little with theabsorber.

As used herein the term optical cavity generally refers to a resonantFabry-Perot-type interferometer comprising at least first and secondmirror structures and the region between such mirror structures. Themirrors and the region between such mirrors may include absorbing, phaseadjusting, sensing, and/or other layers in addition to reflectinglayers. The region between such mirrors also preferably contains avacuum but may comprise solid state layers such as used in photondetectors in accordance with the present invention. The optical cavitiesdescribed herein comprise resonant optical cavities, that is, suchoptical cavities cause constructive interference on resonance within theoptical cavity as opposed to destructive interference. This means thatthe optical cavity spacing between the mirror structures isapproximately a multiple of a half wavelength of the resonantwavelength.

FIG. 1 schematically shows an exemplary device 10 in accordance with thepresent invention having a Gaussian optical cavity 12 and a centralabsorber 14 (sensor) between first and second mirror structures, 16 and18, respectively. Central absorber 14 is positioned in the opticalcavity 12 by supports 20. As shown, mirror 16 comprises a curvedstructure and mirror 18 comprises a planar structure. Any desired mirrorstructure, however, can be used for mirrors 16 and 18 including curvedand planar mirror structures. Mirror reflectivities that complement theabsorption of the central layer are preferably used. As the sensorabsorption decreases, the reflectivities of the mirrors must increase toinsure that the requisite number of passes by the radiation is achieved.So a weaker absorber with properly designed mirrors means a sharperresonance.

FIG. 2 shows an exemplary layer structure for a long-wave infrared(LWIR) cavity that can be used with devices such as thermal detectors inaccordance with the present invention. A simulated spectrum for thelayer structure of FIG. 2 is shown in FIG. 3. The materials used in theexemplary mirrors are germanium and strontium fluoride while the centrallayer is zinc sulfide that has been doped to achieve a desiredabsorption of approximately 0.5% per pass. Such low absorptions are easyto achieve. One can either choose an absorbing material and make thematerial very thin or take a transparent material and dope the materialwith impurities until the desired level of absorption is achieved. As anexample, a single layer of ZnS can be used as a sensor layer inaccordance with the present invention, and functions as sensor,absorber, and structural material. Of course, these functions can beprovided by plural layers.

One way to produce controlled absorption in a semiconductor (e.g. Ge orZnS) that is normally transparent in the mid-wave infrared (3 to 5microns) or long-wave infrared (8 to 14 microns) is to dope it. Whenthis is done, phonons can mediate the interaction of free carriers withincident infrared photons. Because this process involves multipleentities, the resulting absorption coefficients are not as high as thoseof band-to-band transitions, but that is neither desirable nornecessary. The free-carrier absorption has a wavelength dependenceα_(free)˜λ^(P), where p is generally between 2 and 3 depending on whichcombination of acoustic phonons, optical phonons, or impurities mediatethe interaction of carriers with light. The wavelength dependenceproduces a shift in coupling across the LWIR (or MWIR), but does notfundamentally alter performance if the system is optimized for thecenter of the desired band. Other useful layers that are intrinsicallyabsorbing without doping include thin metals such as chromium ordielectrics such as silicon dioxide.

In accordance with the present invention an optical cavity with a weakabsorbing layer positioned therein can be designed to couple essentially100% of incident radiation into the absorber at one wavelength. Thecavity rejects substantially all other radiation. However, this onlyrelates to light along the direction of the optical cavity axis. Alsoimportant to the performance of a thermal detector, is how the radiationis absorbed in other directions. This directional effect can provideabout a factor of 5 in performance enhancement for an f/4 system whilespectral cavity characteristics (directionality and wavelengthselectivity) provide about a factor of 20 for a standard chemical line,for a total of about a factor of 100. Because the absorbing layer can bemade arbitrarily weak, about 1% or less, any radiation that is incidentupon the absorber from directions other than the cavity axis will nothave any additional round-trips of the system; therefore, off-axisradiation is absorbed at only very low levels. In other words,absorption is reduced for all directions and wavelengths except thespecific direction and spectral band that the optical cavity is designedfor.

The Stefan-Boltzmann radiation power equation for a thermal emitter is:

P_(rad)=2AδσT⁴

where A is the area of an element (the factor of two includes top andbottom), ∈ is the emissivity, σ is the Stefan-Boltzmann constant, and Tis the temperature. The radiated power is directly proportional to theemissivity. Because absorptivity is equal to the emissivity byKirchoff's Law, a hypothetical object that is transparent at allwavelengths will not emit any thermal radiation regardless of how hot itis. On the other hand, a weakly absorbing material will emit somethermal radiation, and a strongly absorbing material will emit as ablackbody with ∈˜1 as shown in FIG. 6. According to Kirchoff's Law, anyvariations in the absorption of an object as a function of wavelength,polarization, or direction will also be carried over into the emissionof that object. So fundamental physical law states that a system thatabsorbs in only one wavelength band in one range of directions will onlyemit radiation in that same band and direction.

The concept of low absorption and emissivity is not solely a function ofthe sensor structure itself but also is an aspect of the optical cavityand the sensor position within the optical cavity. A straightforwardexample can explain this. FIG. 4 shows the layer structure of aFabry-Perot cavity with mirrors comprising Ge/SrF2 multilayers. Thecavity is designed for a resonance near 10 microns and the mirrors areseparated by a half-wavelength. The mirrors have high reflectivity butthe bottom mirror reflectivity is nearly 100%. The central layers of thecavity include the innermost high index Ge layers from the mirrors andthe low index vacuum (ignoring the ZnS sensor layer for the moment).Light at the resonant wavelength inside the optical cavity will form astanding wave with an intensity maximum in the center of the opticalcavity (gap between mirror structures). A low-high-low index profile forthe central layers would produce an intensity minimum in the center.Therefore, the radiation will have excellent coupling with any sensorcentered in the optical cavity and only a low value of absorption isneeded to couple most of the radiation into the sensor while rejectingoff-resonance wavelengths. This is illustrated in FIG. 5 with a plot oflight absorbed by the sensor versus wavelength for the detector system.

The standing wave pattern has a minimum near the mirrors, however, so ifthe sensor is placed near the mirrors with an otherwise similar layerstructure as shown in FIG. 19, the ZnS layer must have a much largerimaginary part of the index of refraction in order to achieve goodcoupling. The spectrum is shown in FIG. 20. When absorption layers areplaced close to a mirror one is often better served by materials thatare intrinsically highly absorbing in the IR such as Cr or Pd metalrather than relatively transparent semiconductors as it can be difficultor impossible to dope such materials highly enough to achieve such alarge ‘k’. On resonance, the coupling is still nearly 100% and,off-resonance, the absorption is still nearly zero. Both center-placedsensors and near-mirror-placed sensors can achieve detection beyond thetraditional radiation limit with proper support and read-out design.

To quantify detector performance, noise in a thermal detector needs tobe considered. Assume that the central plate of an optical cavitysimilar to that of FIG. 1 includes a resistor layer of Vanadium Oxide(VO_(x)) and is connected to the outside world by electricallyconducting supports. Noise fluctuations can come from several sources,which include Johnson noise, 1/f noise, and thermal noise (both phononand photon). Johnson noise originates from the interaction of chargecarriers in a resistor with photons from the extreme long wave tail ofthe thermal emission distribution. The Johnson rms voltage noise takesthe form:

√{square root over (

ΔV ²

)}=√{square root over (4kTRB)}

where k is Boltzmann's constant, T is temperature, R is the resistance,and B is the bandwidth of the voltage measurement. To detect a signal,one must receive power that is approximately equal to the noise power.This is codified in a figure of merit called the noise equivalent power(NEP), which is defined as:

NEP = 〈 Δ   V 2 〉

where R_(V) is the detector responsivity in V/W. For a microbolometersensor at peak responsivity frequencies, R_(V)=αV_(b)/G, where α is thetemperature coefficient of resistance (TCR), V_(b) is the bias voltage,and G is the thermal conductance. If we assume a typical TCR ofα=0.02/K, a microbolometer resistance of R=25 kΩ, and a readout bias of0.2V applied for 250 μs once during every frame, then the roomtemperature Johnson noise limited NEP is 0.33 pW.

The second major noise source is 1/f noise. The origin of this noise isnot well understood and is typically different from system to system.The 1/f rms voltage noise takes the form of:

$\sqrt{{\langle{\Delta \; V^{2}}\rangle}\;} = \sqrt{k\; V^{2}\ln \; \frac{f_{2}}{f_{1}}}$

where k is the 1/f noise parameter, f₁ is the time betweenrecalibrations of a bolometer pixel (corresponding to the staring timeor chopping time depending on which mode a bolometer is being used in),and f₂ corresponds to the bandwidth of the electrical readout(f₂˜1/Δt_(read)). In the previous expression for Johnson noise, B=f₂−f₁.All other variables are the same as before. To calculate the 1/fnoise-limited NEP, the previous parameters f₁=30 Hz, and f₂=4 kHz, and atypical VO_(x) value for the 1/f noise parameter, k=10⁻¹³ are used toget an NEP of 0.035 pW.

The most fundamental noise source is thermal conductance noise. Thermalconductance noise originates with the discreteness of the heat transferto and from a highly isolated sensor. The discreteness can arise eitherfrom phonon fluctuations if conduction heat transfer dominates thesystem or photon fluctuations if radiation heat transfer dominates. Ineither case, the noise takes the form:

${NEP} = {G\sqrt{\frac{{kT}^{2}}{C}}}$

where NEP is the noise equivalent power, G is the thermal conductanceand C is the heat capacity of the sensor. (Assume a microbolometerbandwidth of B=¼τ where τ=C/G the time constant for the detector plate.)Radiation heat transfer is usually considered to be the ultimate limitof bolometer performance. In order to estimate the thermal conductanceof a radiation-limited device, we differentiate the Stefan-Boltzmannradiation power law equation and obtain:

G _(rad)=4(2A)∈σT ³

If the size of a microbolometer sensor plate is 100 μm×100 μm, T=300 K,and the emissivity is 1, then G_(rad)=1.5×10⁻⁸ W/K. If the absorption ofthe sensor plate is dropped such that ∈=0.01, then G_(rad)˜10⁻¹⁰ W/K, avalue so low that radiation heat transfer will be overwhelmed byconduction fluctuations from the thermal supports.

The level to which the thermal conductance can be reduced depends on theresistance of the supports. For example, G˜10 ⁻⁹ W/K can be achieved forhigh resistance supports. However, thermal conductances are notnecessarily that low in current uncooled devices because other noisesources such as Johnson noise and the blackbody radiation limit are toohigh to make such low conductance useful.

Assuming that G_(sup) is about 10⁻⁹ W/K. The noise equivalent power fora thermal conductance limited structure is NEP=0.38 pW. Summing thecontributions from thermal, Johnson, and 1/f noise, the NEP=0.50 pW.This level of performance approaches that of the best cooled MCTdetectors. If a single pixel is desired instead of an array (e.g. for achemical detector application rather than imaging) even betterperformance can be obtained by using a high bolometer resistance. A 2.5MΩ resistance allows a large resistance in the support arms, making 10⁻⁹W/K achievable with current materials and standard supports. Read-outspeed is sacrificed, however. The device would need to be read over anentire frame, for example, f₂˜60 Hz and f₁˜30 Hz. The Johnson noise and1/f noise would be about the same or less while the bias voltage can beincreased with less power dissipated into the device, improving the NEP.

Another important figure of merit of microbolometers is the detectivityD*, which normalizes the responsivity to the area and noise of thesensor.

${D^{*} = {R_{v}\sqrt{A}\frac{\sqrt{\Delta \; f}}{V_{n}}}},$

where A is the effective area of the sensor under radiation,V_(n)/√{square root over (Δf)} is the total noise voltage per unitfrequency bandwidth.

For a radiation-limited thermal sensor, the standard fundamentaldetectivity limit is given by:

$D^{*} = \frac{ɛ}{8{ɛ( {T_{1}^{5} + T_{2}^{5}} )}^{\frac{1}{2}}}$

where ∈ is the emissivity, T₁ is the sensor temperature, and T₂ is thebackground temperature. However, this equation assumes an emissivitythat is constant in both direction and wavelength. In FIG. 7 asimulation for a device in accordance with the present invention that isdesigned for a waveband somewhat larger (Δv˜6 cm⁻¹) than a standardatmospheric pressure broadened line (Δv˜4 cm⁻¹). Here the blackbodyfluctuation equation has been integrated over solid angle with anemissivity of nearly one in the direction of an f/4 lens and a muchlower emissivity in other directions. This corresponds to the case of alow absorption material placed in an optical cavity in accordance withthe present invention whose spatial extent matches that of the lens. Inaddition, the spectral range of the optical cavity further reducesinteraction with the background. On resonance, the optical cavitycouples nearly 100% of the light into the sensor. Off resonance, theoptical cavity rejects light back to the background and the mirrorsthemselves provide very little thermal light to the sensor by Kirchoff'sLaw so that a high reflectivity device has a very small emissivity.Mathematically, this means that one integrates Planck's Law (in thedirection of the optical cavity) only over the spectral resonance bandof the optical cavity. This leads to an NEP which goes as the squareroot of the spectral bandwidth. In reality, one multiplies Planck's Lawby the cavity passband absorption but in the limit of narrow resonances,these two methods give similar results. The combination of spectral andspatial effects causes the radiation noise to plummet. FIG. 7 shows thedetectivity of an ultrahigh sensitivity device that is radiation limitedfor a 6 cm⁻¹ resonance in the LWIR. An atmospheric line detector wouldhave a somewhat higher detectivity because of its reduced spectralwidth.

The NEP of a sub-radiation limit sensor is far superior to that of anormal microbolometer with a filter, but its speed can be limited. Thetime constant of a microbolometer is τ=C/G where C is the heat capacityand G is the thermal conductance. Since reducing the thermal radiationlimit reduces the thermal conductance, the heat capacity must be reducedin order to maintain rat a normal frame rate of 30 Hz. This can be doneby reducing the size of the pixel. Because the beam has to make multipleround trips of the cavity, there can be significant diffraction andcoupling losses if the pixel size becomes comparable to the wavelengthin a parallel plate cavity. There are a few ways to circumvent thisproblem. The simplest is to maintain pixel size but reduce the thicknessof the bolometer membrane (sensor) to nanometer-scale thicknesses. Thisrequires control of thin film stress but does not required a change inarchitecture. The second is to use a Gaussian optical cavity that ismatched to produce a spot size that covers the bolometer membrane. Thecavity focuses the light and thus the bolometer area can besignificantly smaller. In such a design, diffraction will occur but themirrors of the cavity are shaped (curved) to match the diffractedwavefront and redirect the diffracted wavefront back towards thebolometer membrane. This type of cavity is used in many lasers to matchthe beam size to the gain region. The mirrors can include stress-shapedmirrors, tunable mirrors, coated microlenses or the like. A third methodof circumventing diffraction losses is to alter the bolometer membraneto become a sub-wavelength grating. This allows significant mass to beremoved from the bolometer membrane without having to reduce the areaand/or thickness of the bolometer membrane. The optical properties ofsub-wavelength gratings can be altered for a desired amount ofabsorption in the same way as for reflection or transmission.

Any one or some combination of these may be used in sensor structures inaccordance with the present invention. For example, assume a 10 micronbolometer membrane in a matched Gaussian cavity. If the thickness of theplate is 50 nanometers, and the system is made of normal materials suchas silicon nitride, then the heat capacity will be on the order of 10⁻¹¹J/K, which if used with G of about 10⁻⁹ W/K, would maintain a timeconstant of τ=10 ms. If the detector size were increased to 35 microns,the plate would preferably be thinned in some way or made into agrating.

One difficulty with achieving low thermal conductance in microbolometersupports is the need to maintain high electrical conductance.Simultaneously high electrical conductivity, σ, and low thermalconductivity, κ, is difficult to achieve and limited for conductors bythe Wiedemann-Franz Law. Some of the materials with the highest ratio ofσ/κ are the thermoelectrics and some metal alloys such as NiFe. In thenoise calculations set forth above, the resistance of a single-pixelbolometer was assumed to be 2.5 MΩ. The supports for such a device couldhave a resistance up to about 100 kΩ or more per support before anysignificant loss in signal would be seen. Assuming a silicon dioxidesupport with a NiFe conductor (using relatively standard values forthese materials), a 2 mm long beam with about a 0.55 μm² cross-sectionalarea would have a thermal conductance of 5×10⁻¹⁰ W/K.

In a preferred embodiment of the present invention, an electrical (oroptical) read-out operation occurs over the entire frame time tominimize bandwidth. If one wishes to sequentially read an entire row orcolumn in an array during a single frame, say by using pulse bias,detection beyond the standard limit becomes more difficult. Because theelectrical bandwidth increases, the Johnson noise increases, and the NEPdegrades significantly. To avoid this, a smaller resistance plate can beused, say R_(plate) of about 25 kΩ with supports of R_(sup)˜1 kΩ, butthe achievable thermal conductance will be limited to a value on theorder of 10⁻⁸ W/K for existing materials. There are a few ways tocircumvent this problem; one technique is to create a thermal switchthat only contacts the bolometer during electrical read-out. Thermalswitches have been proposed and demonstrated in thermal detectors forchopping and responsivity control. Thermal switching to latch anelectrical connection for thermal isolation during an electrical offcycle is used with thermoelectric coolers. The use of thermal switchingto latch an electrical connection for thermal isolation during anelectrical off cycle can be applied to microbolometers. The pixels areprovided with extra leads, higher voltages, and voltage controlcircuitry. Stiction issues are addressed using a two-voltage-stepactuation approach.

An exemplary thermal detector 22 in accordance with the presentinvention is shown schematically in FIG. 8. Thermal detector 22, asshown, includes first mirror structure 24, second mirror structure 26,and sensor structure 28 supported within optical cavity 30 by supportbeams 30 and 31. Sensor structure 28 includes sensor portion 29 andsupport structures 30 and 31. Second mirror structure 26 includesaperture 32 that allows incoming radiation to pass through second mirrorstructure 26 and enter optical cavity 30.

An exemplary process for making thermal detector 22 preferably beginswith three substrates. Known semiconductor processing techniques andMEMS fabrication techniques can be used to manufacture thermal detectorsin accordance with the present invention. For process simplicity, anon-monolithic structure is illustrated and described. In someembodiments, one or more of the wafers can be eliminated by stackingmore dielectrics. Silicon is chosen for its cost-effectiveness andwidespread use, but other materials, such as germanium or quartz can beused as well depending on process and wavelength considerations. Anexemplary sensor structure 28 is shown in FIG. 9 in cross-section and atop view is shown in FIG. 10. Sensor structure 28 includes sensorportion 29 and support structures 30 and 31. The sensor structure 28includes a first dielectric layer 34, an absorbing layer 36, a seconddielectric layer 38, a TCR (sensor) layer 40, and substrate 42. Some orall of these layers could be removed, combined (e.g. a common layer forsensor and absorption, or dielectric and absorption), or added to (e.g.a protective dielectric could be put over the TCR material). It isassumed in the exemplary process that the sensor and absorber layers areuniform across the device. However in some alternatives, the sensor andabsorber can be patterned so long as the combined emissivity of thecombined sensor/absorber structure is specifically designed to be lessthan the standard blackbody radiation limit and is absorption-coupled tothe optical cavity in the direction of the optical signal (while notbeing optimally coupled in most or all other directions). In oneexample, the dielectric material is silicon dioxide which also functionsas the absorber, while the TCR material is vanadium oxide (VOx). The VOxhas been chosen because it is a current industry standard, but anynumber of other TCR materials can be used. The sensor is patterned andthen support layers are deposited and patterned. The supports aredesigned to have very low thermal conductance and, in the illustratedembodiment, a first layer 44 of silicon dioxide and second layer 46 ofNiFe are provided on layer 34 used as the structural andelectrically-conductive layers respectively for support beams 30 and 31.Two support beams are illustrated but any of support beams such as one(with two NiFe lines) or three or four (where one or more are SiO₂ only)can be used.

Once the sensor and supports are complete, the substrate underneath thedevice region is etched away by wet or dry etching, for example by aconventionally known Bosch process. The removal of the portion of thesubstrate beneath the sensor in this embodiment prevents unwantedreflections and silicon absorption. If these processes require etch stopor protective layers, then these can be added to the device depositionsabove. The Bosch process, for example, may only need the SiO₂ dielectricas an etch stop.

Referring to FIG. 11, second mirror structure 26 and sensor structure 28are shown in exploded view. Second mirror structure 26 includesdistributed Bragg reflector (DBR) structure 48 deposited on a substrate50 along with spacer(s) 52 of appropriate thickness. The spacer(s) 52are patterned to surround the device region of the sensor structure 28while still allowing access to the electrical leads of the sensorstructure 28. The DBR structure 48 may include, for example, Ge and ZnSquarter-wavelength layers. It should be noted that for some devices,better spectral characteristics (e.g. improved symmetry in an absorptionversus frequency plot) might be obtained by using non-quarter wavelayers. The spacer(s) 52 can be bonded to the substrate 50 using any oneof a number of semiconductor processes, for example, thin epoxy. Thespacer(s) 52 are used to define a desired distance between the secondmirror structure 26 and the sensor. In most cases the spacing is used tohelp define an overall half-wavelength or multiple-of-a-half-wavelengthoptical resonant cavity. As noted above, the substrate underneath thedevice region is preferably removed by a wet or dry etch (for examplethe Bosch process, which would require an extra SiO₂ etch-stop layer andits subsequent removal) to eliminate substrate reflections andabsorption.

The first mirror structure 24 shown in FIG. 12 is also preferably coatedwith a DBR 54 or similar high-reflectivity coating. In many cases, thereflectivity of the first mirror structure 24 will be chosen to benearly 100% to minimize losses, minimize unwanted coupling to directionsother than that of the signal, and extend the useful tuning bandwidth inthe case of tunable devices. DBR 54 is preferably patterned and thesubstrate region around it is over-etched to a pre-determined depth. Thefirst mirror structure 24 can then be inserted into a correspondingregion 56 underneath the sensor structure 28 so that the first mirrorstructure 24 and sensor structure 28 are at a well-defined spacing.Again, this spacing is preferably chosen to define an overall cavitylength (including all structures) of a half-wavelength ormultiple-of-a-half-wavelength of the desired resonant frequency (orstarting resonant frequency in the case of tunable devices).

In a thermal detector, the top mirror and sensor layer can be combinedand still have performance beyond the radiation noise limit. Thisrequires that the absorber and sensor combination be specificallydesigned for low emissivity compared to the blackbody radiation limitfor the area of the device and that the supports and read-out bedesigned to reduce other noise sources below the blackbody radiationlimit. Such an architecture is described in U.S. patent application Ser.No. 11/805,240 to Talghader et al., and would look similar to thedevices described therein except with the supports and read-out designedfor much lower noise. However, there are performance aspects related tothis. First, the extra volume of the top mirror when combined with theabsorber/sensor may increase the time constant. Methods to mitigatethis, such as by thinning the top mirror by using special metals anddielectrics as described in U.S. patent application Ser. No. 11/805,240,may result is less optimal absorption characteristics off axis andoff-resonance. Second, the two-layer optical cavity may allow more lightfrom directions other that the optical axis, possibly reducingefficiency. That being said, significant NEP improvement can still beachieved.

As noted previously, the sensor plate itself can be placed at a varietyof points within the optical cavity, depending on the optical coatingdesign. For example, if the sensor is equidistant between the twomirrors, a lightly doped semiconductor layer may work well as theabsorber and/or TCR material. If the sensor plate is close to the top orbottom mirrors, a thin metal layer may work well for optimal coupling.

A case of particular interest is when the sensor is near the flat bottommirror and the cavity has a curved top mirror as shown in thermaldetector 60 of FIG. 13. Thermal detector 60 is similar to thermaldetector 22 shown in FIG. 8 and includes first mirror structure 24,sensor structure 28, second mirror 62, respectively, and spacers 68.Second mirror structure 28 may comprise any layered mirror structuresuch as those described herein. In this case, if the curvature of thesecond mirror structure 62 is set to form a half-spherical cavity withthe flat first mirror structure 24 (i.e. the radius of curvature of thesecond mirror structure 62 matches the distance between the first andsecond mirror structures, 24 and 62, respectively) then lower f/# opticsmay be matched to the optical cavity than are otherwise possible withflat mirror cavities. A normal flat-flat mirror cavity that is sensing astandard atmospheric chemical line of spectral width 4 cm⁻¹ might use anf/4 lens at best before the angular range of the input signal begins tosignificantly degrade the spectral resolution of the sensor. The usablef/# and therefore the NEP can be lowered significantly by matching theangular spread of the rays (i.e. the shape of the wavefront) through thelens to the radius of curvature of the second mirror structure 62. Theplacement of the sensor near the first mirror structure 24 insures thatthe over all propagation distance of any ray in the cavity is nearly thesame. More particularly, the sensor is spaced from first mirrorstructure 24 by a distance corresponding a small fraction of awavelength, for example, less than λ/10. The use of the curved mirror,low f/# optics, and proper sensor placement allows the sensor to bereduced in size relative to the resonant wavelength as compared toflat-flat mirror cavities.

In FIG. 14 another exemplary thermal detector 70 in accordance with thepresent invention is schematically shown. Thermal detector 70 includesfirst and second mirror structures, 72 and 74, respectively, and sensorstructure 28. First mirror structure 72 comprises multilayer stack 76and spacers 80. Second mirror structure 74 comprises multilayer stack 82and spacers 86. The performance of the curved-curved cavity isrelatively similar to the curved-flat cavity except for an increasedminimum length, which may introduce other resonances into the spectralbands of the DBR mirrors. Such a mirror can be used to simplifymanufacturing processes as the top and bottom mirrors could be nearlyidentical in optics and electrode structure and thus diced from the samewafers.

Thermal detectors in accordance with the present invention can also bemade tunable. This can be done in many ways with many types ofmicromechanical actuators, such as piezoelectric, electrostatic, andmagnetic for example. An exemplary electrostatic based thermal detector88 in accordance with the present invention is shown in FIG. 15 and issimilar to the thermal detector 60 shown in FIG. 13. FIG. 16 shows a topview of detector 88 and FIG. 17 shows a top view of sensor structure 28.The process remains largely the same but would include extra steps forpatterning actuators and supports for the top mirror. Actuationelectrodes provided on the supports are optional. The actuator voltageV2 could be applied directly to the sensor element as shown or withseparate electrodes. If applied directly to the TCR layer and/orabsorber layers then the read-out bias would need to be considered inapplying voltages. For example, if the actuation voltages(V1−V2)=(V3−V2)=20 V but 0.02 V were required to bias the device forread, then one side of V2 would be set at 0 V and the other would be setat 0.02 V, while V1 and V3 would be set at 20 V. The potentialdifferences V1−V2 and V3−V2 are not in general the same, and theirmagnitude and timing can be adjusted individually to maintain optimalspectral characteristics. These actuation voltages may also be eitherapplied continuously or dynamically (e.g. timed steps or pulses). Incertain cases, layers in the top and bottom mirrors and the sensor maybe used as actuation electrodes. For example, the surface Ge layerfacing the gap in the mirrors and the TCR layer in the sensor can beused as actuation electrodes.

In most cases, case, it is preferable that dielectric not come betweenelectrodes as this may lead to undesired dielectric charging. Regions ofthe sensor structure may also have a metal electrode underneath thedielectric that may be directly connected to the sensor top actuationelectrode. Also note that in order to read-out the detector, a potentialdifference will need to be placed across the left and right electrodesof the sensor. This can cause a slight tilt in the position of thedetector relative to the top and bottom mirrors. However, the potentialdifference will likely be very small relative to the actuation voltagesand therefore the tilt will not affect optical performance except atextremely high cavity finesse.

Although it is expected that most applications of detection beyond theradiation limit will be for uncooled thermal devices in the infrared,the concept can equally well be applied to photon detectors in theinfrared. Fundamentally the present invention reduces the interaction ofthe thermal background with narrowband detectors, so any detector thatis background limited can use the concepts herein. In a photon detector,one will usually cool the detector to a level where the limiting noiseis the thermal background. If one designs the electron-hole absorbingregion of the detector to have a small absorption per pass and placesthis system in the cavity, the background limit is reduced. Therefore inorder to retain background-limited performance, one would have tofurther cool the device so that the device noise is below the newbackground limit. Under normal circumstances, this process is littledifferent from a standard cold-shielded and cold-filtered detector, butwith the development of microcoolers that can cool very small volumes,one could encounter electron-hole generation regions that are cooled totemperatures much smaller than the system around them. (Or alternativelythe regions around them become heated in some way.) This type of photondetector could certainly benefit from the techniques described here.

An exemplary photon detector 90 in accordance with the present inventionis schematically shown in FIG. 18. Detector 90 includes substrate 92having first mirror structure 94 and optional microcooler 96. Activeregion 98 is provided on first mirror structure 94. Active region 98comprises n-type layer 100 and p-type layer 102 having interface region104 therebetween. Interface region comprises a low bandgap absorbingelectron-hole generation region. Photon detector 90 also includes secondmirror structure 106 and first and second contacts, 108 and 110. Thesensor in this embodiment includes the absorbing electron-holegeneration region 104. The optical cavity include the region betweenmirrors 94 and 106 and including mirrors 94 and 106. Photon detector 90operates in much the same manner as a normal photodetector in thatinfrared light is absorbed by the electron-hole-pair generation region104, except in this device, only light on the cavity resonance issignificantly absorbed. In this device, the region around the sensor ispreferably cooler than the surrounding components and thus a reducedabsorption in the cavity leads to a reduced radiation limit. Photondetector 90 can be made tunable by introducing a gap and electrodesbetween one mirror and the remainder of the device.

Thermal detectors in accordance with the present invention can be usedin chemical sensing devices, industrial process control devices, enginemonitoring devices, effluent monitoring devices, environmentalmonitoring devices, temperature measurement devices, pressuremeasurement devices, explosives detection devices, imaging devices,medical monitoring devices, for example. Such detectors may, forexample, replace any filter and infrared detector, monochromator andinfrared detector, or grating/diffractive structure and infrareddetector in such systems. The operational concept for these applicationsis that emission/absorption spectra being read has variations in itsspectral intensity that can be measured using detectors in accordancewith the present invention and converted into a relevant signal that,for example, describes intensity, intensity ratio, or linewidth toassess chemical concentration, temperature, or pressure.

Other embodiments of this invention will be apparent to those skilled inthe art upon consideration of this specification or from practice of theinvention disclosed herein. Various omissions, modifications, andchanges to the principles and embodiments described herein may be madeby one skilled in the art without departing from the true scope andspirit of the invention which is indicated by the following claims.

REFERENCES

The following references are each incorporated by reference herein fortheir entire disclosure and for all purposes.

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1. A thermal detector for sensing infrared radiation, the thermaldetector comprising: a resonant optical cavity at least partiallydefined by first and second spaced apart layered thin-film mirrorstructures; and an infrared sensor operatively positioned between thefirst and second thin-film mirror structures and suspended within theresonant optical cavity by one or more electrically conductive supportstructures so radiation received by the resonant optical cavity impingeson at least a surface portion of the infrared sensor.
 2. The thermaldetector of claim 1, wherein at least one of the first and secondthin-film mirror structures comprises a Distributed Bragg Reflector. 3.The thermal detector of claim 1, wherein the first and second thin-filmmirror structures provide a standing maximum centered in the resonantoptical cavity.
 4. The thermal detector of claim 1, wherein at least oneof the first and second thin-film mirror structures comprises areflectivity centered between about 3 microns and about 14 microns. 5.The thermal detector of claim 1, wherein the resonant optical cavityprovides a vacuum to thermally isolate the infrared sensor from thefirst and second thin-film mirror structures.
 6. The thermal detector ofclaim 1, wherein the infrared sensor comprises a bolometer.
 7. Thethermal detector of claim 6, wherein the bolometer comprises a thin-filmtemperature sensitive resistor electrically connected to the one or moreelectrically conductive support structures.
 8. The thermal detector ofclaim 7, wherein the thin-film temperature sensitive resistor comprisesone or more of vanadium oxide, YBaCuO, polysilicon, and titanium.
 9. Thethermal detector of claim 6, wherein the bolometer comprises anabsorbing layer.
 10. The thermal detector of claim 9, wherein theabsorbing layer comprises one or more of a metal, alloy, dielectric, andsemiconductor.
 11. The thermal detector of claim 1, wherein the one ormore electrically conducting support structures thermally isolate theinfrared sensor and comprises a thermal conductance per unit surfacearea below about 1 W/(Km²).
 12. The thermal detector of claim 1 incombination with one or more of a chemical sensing device, an industrialprocess control device, an engine monitoring device, an effluentmonitoring device, an environmental monitoring device, a temperaturemeasurement device, a pressure measurement device, an explosivesdetection device, an imaging device, and a medical monitoring device.13. A thermal detector for sensing infrared radiation, the thermaldetector comprising: a resonant optical cavity at least partiallydefined by first and second spaced apart layered thin-film mirrorstructures; and an infrared sensor operatively positioned between thefirst and second thin-film mirror structures and suspended within theresonant optical cavity by one or more electrically conductive supportstructures so radiation received by the resonant optical cavity impingeson at least a portion of the infrared sensor and wherein said portion ofthe infrared sensor has an absorption less than or equal to twenty-fivepercent.
 14. The thermal detector of claim 13, wherein the infraredsensor has an absorption less than about ten percent.
 15. The thermaldetector of claim 13, wherein the infrared sensor comprises a bolometer.16. The thermal detector of claim 13, wherein the one or moreelectrically conducting support structures thermally isolate theinfrared sensor and comprises a thermal conductance below about 1W/K/m².
 17. The thermal detector of claim 13, wherein the infraredsensor comprises a membrane having a thickness less than about onehundred nanometers.
 18. The thermal detector of claim 13, wherein theone or more electrically conducting support structures comprises athin-film structure comprising an insulating layer and an electricallyconductive layer.
 19. A method of sensing infrared radiation, the methodcomprising the steps of: providing a resonant optical cavity at leastpartially defined by first and second spaced apart mirror thin-filmstructures and an infrared sensor suspended within the resonant opticalcavity; causing radiation to enter the resonant optical cavity andcouple with the infrared sensor; and measuring a change in the infraredsensor caused by the radiation coupled with the infrared sensor.
 20. Themethod of claim 19, therein the infrared sensor comprises a bolometer.21. The method of claim 19, wherein the step of measuring a change inthe infrared sensor includes measuring a change in the resistance of atleast a portion of the infrared sensor.
 22. A method of making a thermaldetector for sensing infrared radiation, the method comprising the stepsof: forming a resonant optical cavity at least partially defined byfirst and second spaced apart thin-film mirror structures; suspending aninfrared sensor within the resonant optical cavity; and positioning theinfrared sensor within the optical cavity so the absorption of theresonant wavelength of the optical cavity by the sensor is greater thantwenty-five percent.
 23. The method of claim 22, comprising providing avacuum in the resonant optical cavity.
 24. The method of claim 22,wherein the step of suspending an infrared sensor within the resonantoptical cavity comprises forming a bolometer structure.
 25. The methodof claim 22, wherein the absorption of the resonant wavelength of theoptical cavity by the sensor is greater than seventy-five percent.